Takagi water heaters do not have tanks, instead they heat water instanteously using a gas burner. They are efficient and powerful — even the smallest heater, the Takagi T-KJr., can produces 180 gallons of hot water in an hour, but weighs only 30 pounds and is 20 inches high. They run on propane or natural gas.
Computer controlled electronic ignition allows the heater to operate without a pilot light and save up to 50% off utility costs. Prices range from $600-$1000. The heaters also have powered ventilation, computer-modulated gas flame for steady output temperatures and the industry’s best energy factor of 0.84
Takagi heaters can be used with solar preheated incoming water — only adding additional heat as needed to meet a set output temperature.
Here’s a short new story on Takagi heaters: ABC News